Windows Phone 8 Unit Test creating a bitmapimage

Question

I have a method that takes a BitmapImage.

I am trying to test it by creating or loading a BitmapImage and then passing it to said method.

However, unit test does not allow me to create a bitmapimage, it throws an InvalidCrossThreadException.

Is there any documentation or resource that details how to unit tests methods that take BitmapImages in Windows Phone 8.

We are using Visual Studio 2012 - update 2.

Answer 1

BitmapImage can only run on the UI thread, and the Unit-Test is running from a background thread. This is why you are getting this exception. For any tests involving BitmapImage or other UI component you'd need to:

1. Push the UI work to the UI thread using Dispatcher.BeginInvoke()
2. Wait for the UI thread to finish before completing the test.

For example, using a ManualResetEvent (semaphore) to do the cross-thread signalling, and making sure that any (catchable) exceptions are passed back to the test thread...

[TestMethod]
public void TestMethod1()
{
    ManualResetEvent mre = new ManualResetEvent(false);
    Exception uiThreadException = null;

    Deployment.Current.Dispatcher.BeginInvoke(() =>
        {
            try
            {
                BitmapImage bi = new BitmapImage();

                // do more stuff
                // simulate an exception in the UI thread
                // throw new InvalidOperationException("Ha!");
            }
            catch (Exception e)
            {
                uiThreadException = e;
            }

            // signal as complete
            mre.Set();                    
        });

    // wait at most 1 second for the operation on the UI thread to complete
    bool completed =  mre.WaitOne(1000);
    if (!completed)
    {
        throw new Exception("UI thread didn't complete in time");
    }

    // rethrow exception from UI thread
    if (uiThreadException != null)
    {
        throw uiThreadException;
    }
}